In the U.S. Pat. No. 5,010,063 a description was given of a structural modification, in basic medium, of glycosaminoglycans with heparin and heparan structure with subsequent isolation from the reaction mixture of new derivatives with respect to the state of the art, as demonstrated unmistakably by the chemical and physical characteristics and especially by the .sup.13 C-NMR spectrum.
In the subsequent U.S. Pat. No. 5,104,860 a further structural modification was described, in a basic or neutral medium, which, starting from the products formed in the reaction conditions described in U.S. Pat. No. 5,010,063, and from the glycosaminoglycans with heparin or heparan structure used as starting products in the same patent, originated a range of new products, different from those described in said patent and new with respect to the state of the art, as demonstrated unmistakably by the chemical and physical characteristics and especially by the .sup.13 C-NMR spectrum.
The chemical and physical characteristics of the products described in U.S. Pat. No. 5,010,063 and the results of a subsequent structural study described by Jaseia M., Rej R., Sauriol F., Perlin A. S. in Can. J. Chem 67, 1449-56 (1989), with the specific aim of explaining the mechanism of the reaction of structural modification in a basic medium, have demonstrated that these derivatives show a modification which concerns just one of the monosaccharide units characteristic of glycosaminoglycans with heparin or heparan structure, more specifically the unit of .alpha.-L-iduronic acid sulfated in position 2 and involving its transformation into a 2,3-epoxygulonic unit. The so obtained epoxides are represented from the following general formula IV ##STR2##
Likewise it has been demonstrated that semi-synthetic glycosaminoglycans with one 2,3-epoxygulonic unit and also glycosaminoglycans with heparin or heparan structure, in conditions of reaction similar to those described in U.S. Pat. No. 5,104,860, undergo a structural modification which also concerns the saccharide unit of .alpha.-L-iduronic acid sulfated in position 2 and involving the transformation of this saccharide unit into a unit of non-sulfated .alpha.-L-iduronic acid or .alpha.-L-galacturonic acid, according to the conditions of reaction used.
So U.S. Pat. No. 5,010,063 describes semi-synthetic glycosaminoglycans containing an epoxy function between positions 2 and 3 of the unit of .alpha.-L-iduronic-2-O-sulfate acid taken as a starting point and the conditions of reaction necessary for obtaining them, while U.S. Pat. No. 5,104,860 describes products deriving from further transformation of the epoxide, confirmed as having one unit of non-sulfated .alpha.-L-iduronic or .alpha.-L-galacturonic acid, and the conditions of reaction necessary for obtaining them starting from the epoxide itself or, as an alternative, starting from the glycosaminoglycans with heparin or heparan structure themselves, used as starting products in U.S. Pat. No. 5,010,063.
Subsequently, in the published European patent application EP 565.863 a description was given of semi-synthetic glycosaminoglycans in which one of the saccharide units characteristic of glycosaminoglycans with heparin or heparan structure, more specifically that containing .alpha.-L-iduronic-2-O-sulfate acid, has undergone, totally or partly, a structural modification in position 2, position in which the sulfate group has been substituted with a nucleophilic group. The process claimed in said published European patent application describes the obtaining of the semi-synthetic glycosaminoglycans of general formula III ##STR3## by treating the epoxides of formula IV, described in U.S. Pat. No. 5,010,063 with a nucleophilic agent.
Object of the present invention is a new process for the preparation of semisynthetic glycosaminoglycans of general formula III directly starting from the glycosaminoglycans with heparin or heparan structure of general formula I ##STR4## This new process represents an improvement of the process described in the published European patent application EP 565.863 because it uses as starting product the glycosaminoglycan of formula I while in said published European patent application the starting product was the epoxide derivative of formula IV obtained in its turn from the glycosaminoglycan of formula I according to the process described in U.S. Pat. No. 5,010,063. The advantage of directly obtaining the product of formula III in only one reaction by starting from the glycosaminoglycan of formula I instead of obtaining it by means of two consecutive reactions, the first of which includes the process of synthesis, isolation and purification of the epoxide of formula IV starting from the glycosaminoglycan of formula I, is evident as regards the overall yield and the industrial cost.
To better define the field of the present invention, we would like to point out that the expression glycosaminoglycans with heparin or heparan structure is intended to indicate polysaccharides with a molecular weight of between about 3000 and about 50000 Daltons and characterized by the fact of possessing a disaccharide unit consisting of a uronic acid (which may be .alpha.-L-iduronic or .beta.-D-glucuronic) and of .alpha.-D-glucosamine, connected, in alternate sequences, by 1,4-glycosidic bonds as described by Lindhal U., Kjellen L. in Thrombosis and Haemostasis 66, 44-48 (1991) and by Turnbull J. E., Gallagher J. T. in Biochem. J. 273,553-559 (1991). Since the .alpha.-L-iduronic acid can be sulfated in position 2 and the glucosamine can be N- acetylated, N-sulfated, 6-O-sulfated, 3-O-sulfated, according to the variable positions of the substituents, at least 10 different disaccharide units are possible, whose combination may generate a large number of different sequences. Bearing in mind the most represented disaccharide units and the most frequent sequences, we can say with reasonable approximation, that the general formula I can be attributed to glycosaminoglycans with heparin or heparan structure ##STR5## where R represents hydrogen or the sulfate residue (SO.sub.3.sup.-) and where m and n are whole numbers between 1 and 100.
In heparin structured glycosaminoglycans of natural origin the value of m is high and the disaccharide unit A represents about 80% of the disaccharide units; on the contrary, in heparan structured glycosaminoglycans of natural origin the value of n is high and the disaccharide unit B represents about 80% of the disaccharide units.
The general formulae I and III are intended to reveal the composition of the main saccharide units but make no reference to their sequence.
As is known to experts in the art, it is possible to make a chemical modification of glycosaminoglycans of natural origin, for example through reactions of N-desulfatation, possibly followed by reactions of N-acetylation, thus also obtaining semi-synthetic N-desulfated heparins or N-desulfated-N-acetylated heparins. In addition, these glycosaminoglycans, whether natural or semi-synthetic, may be subjected to depolymerization processes by means of which the molecular weight is taken to levels generally between 3000 and 10000 Daltons.
The structural modification described in this invention for obtaining new semi-synthetic glycosaminoglycans with heparin or heparan structure refers to the unit of .alpha.-L-iduronic-2-O-sulfate acid where the partial or total selective substitution of the O-sulfate group in position 2 with a nucleophilic residue, whatever it might be the desired compound with heparin or heparan structure, takes place. Indeed, besides being selective, the chemical process described in this invention can be applied to glycosaminoglycans with heparin or heparan structure which present all the possible sequences; ie. it is independent of the type and of the level of functionalization of the saccharide unit which precedes or follows in the sequence the unit of .alpha.-L-iduronic-2-O-sulfate acid which is the object of the reaction of structural modification.
The structure of the new products is represented by the general formula III ##STR6## where p+q=m, with p other than 0, and m, n and R have the meaning as seen above, and where --Z(R.sub.2)R.sub.1 represents the nucleophilic group introduced through the process described in this invention.
The reaction of structural modification which involves the partial or total introduction of the nucleophilic group in position 2 of the .alpha.-L-iduronic acid does not lead to the depolymerization of the glycosaminoglycans or alteration in the distribution of the molecular weight of the polysaccharide chains which form them, and for this reason the present reaction can be applied to glycosaminoglycans with heparin or heparan structure of any molecular weight. The products obtained can however be subjected to the known processes of chemical or enzymatic depolymerization.